This long-standing project studies virus and host factors that influence the pathogenesis of neoplastic and non-neoplastic diseases induced by murine leukemia viruses (MuLV). MuLVs so studied have included the Moloney and Friend ecotropic (mouse cell-tropic) isolates, mink cell focus-inducing (MCF) viruses from AKR mice and other sources, ecotropic and amphotropic viruses isolated from wild mice and the defective BM5 immunodeficiency MuLV. At present the major emphasis is on characterizing and determining the pathogenic potential of viruses isolated from B cell and T cell lymphomas occurring in NSF.V+ mice. These are NFS mice congenic for ecotropic MuLV genes derived from AKR and C58 mice, both strains having high expression of virus and high incidence of T cell lymphoma. From one B cell lymphoma a virus preparation was isolated that contains both ecotropic and MCF MuLV classes and which induces lymphoblastic lymphoma, ususally B cell but occasionally T cell, upon inoculation into newborn NIH Swiss mice. The latency is relatively long, the earliest tumors developing at about 3 mo post inoculation. Although molecular analysis did not reveal any viral component other than full-length ecotropic and MCF MuLVs, disease has been induced only with the original virus mixture or several uncloned preparations from lymphoma-bearing recipients, not with cloned viruses alone or mixed.